Chishiro Michioka

Unconventional superconductivity and nearly ferromagnetic spin fluctuations in NaxCoO2.yH2O

Co nuclear-quadrupole-resonance (NQR) studies were performed in the recently discovered superconductor Na x CoO 2 · y H 2 O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same Na x CoO 2 were examined, a SC bilayer-hydrate sample with T c ∼4.7 K and a non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate 1/ T 1 in the SC sample, it was found that the coherence peak is absent just below T c and that 1/ T 1 is proportional to temperature far below T c . These results, which are in qualitative agreement with the previous result by Fujimoto et al. , suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, 1/ T 1 T of the SC sample shows a gradual increase below 100 K down to T c , whereas 1/ T 1 T of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between 1/ T 1 T and χ bulk in the SC sample, the increase in 1/ T 1 T is at...

59Co Nuclear Quadrupole Resonance Studies of Superconducting and Nonsuperconducting Bilayer Water Intercalated Sodium Cobalt Oxides NaxCoO2·yH2O

We report 59 Co nuclear quadrupole resonance (NQR) studies of bilayer water intercalated sodium cobalt oxides Na x CoO 2 · y H 2 O (BLH) with the superconducting transition temperatures, 2 K < T c ≤4.6 K, as well as a magnetic BLH sample without superconductivity. We obtained a magnetic phase diagram of T c and the magnetic ordering temperature T M against the peak frequency ν 3 of the 59 Co NQR transition I z = ±5/2 ↔±7/2 and found a dome-shaped superconducting phase. The 59 Co NQR spectrum of the nonsuperconducting BLH shows a broadening below T M without the critical divergence of 1/ T 1 or 1/ T 2 , suggesting an unconventional magnetic ordering. The degree of enhancement of 1/ T 1 T at low temperatures increases with the increase of ν 3 though the optimal ν 3 of approximately 12.30 MHz. In the Na x CoO 2 · y H 2 O system, the optimal- T c superconductivity emerges close to the magnetic instability. T c is suppressed near the phase boundary at ν 3 ∼12.50 MHz, which is not a conventional magnetic quantu...

Magnetic Properties of Layered Itinerant Electron Ferromagnet Fe3GeTe2

We have successfully synthesized a single-crystal sample of the layered structure ferromagnet Fe3GeTe2 and present the results of its complete characterization using powder X-ray diffraction (XRD), magnetic susceptibility χ(T), isotherm magnetization \(M(H)\), specific heat \(C(T)\), and electrical resistivity ρ(T) measurements. The rietveld refinement of the powder XRD data confirms that Fe3GeTe2 crystallizes in the space group \(p6_{3}/mmc\). The χ(T), \(C(T)\), and ρ(T) data indicate bulk ferromagnetic ordering at \(T_{\text{C}} = 220\) K with a saturation moment of 1.625 µB/Fe at zero temperature. The magnetic measurements of the external magnetic field \(H\) applied along the \(c\)-axis and \(ab\)-plane show a large anisotropy in this material. We found that the modified Arrott plot (\(M^{4}\)–\(H/M\)) rather than the \(M^{2}\)–\(H/M\) plot is obeyed in this material. The generalized Rhodes–Wohlfarth ratio indicates an itinerant magnetism in this material. The spin fluctuation parameters are estimate...

Observation of Bose-Einstein Condensation of Triplons in Quasi 1D Spin-Gap System Pb2V3O9

We observed a field-induced magnetic ordering in the quasi one-dimensional antiferromagnetic alternating chain system Pb 2 V 3 O 9 for the first time. The characteristic temperature dependence of t...

Coexistence of Superconductivity and Ferromagnetic Spin-Fluctuation in Ca3Ir4Sn13 Single Crystal

Temperature dependences of the electrical resistivity, magnetic susceptibility and specific heat have been measured on the single crystal Ca 3 Ir 4 Sn 13 . This compound shows a bulk superconducting transition at ≈7.0 K. We observe peak-like anomalies, which are attributed to the ferromagnetic spin-fluctuations, in susceptibility and resistivity. The resistivity shows a non Fermi-liquid behavior as ρ=ρ 0 + A * T n ( n =1.2 for H =0) in normal state at low temperatures, and develops into a Fermi-liquid behavior by the suppression of the ferromagnetic spin fluctuation on increasing magnetic fields ( n =2 for H =14 T). The superconductivity disappears for magnetic field H ≥7 T as well as the diminution of the ferromagnetic spin fluctuations. We find that the superconductivity coexists with the ferromagnetic spin fluctuations and seems to cooperate in this material. The large values of Δ C /γ n T c =2.78, Kadowaki–Woods ratio A /γ n 2 =1.14×10 -5 µΩ·cm (K·mol/mJ) 2 and Wilson ratio R w =π 2 k B 2 χ p /3µ B 2 ...

Unconventional superconductivity induced by quantum critical fluctuations in hydrate cobaltate Nax(H3O)zCoO2 ·yH2O - : Relationship between magnetic fluctuations and superconductivity revealed by co nuclear quadrupole resonance

Co nuclear-quadrupole-resonance (NQR) measurements were performed on various bilayered hydrate cobaltate Na x (H 3 O) z CoO 2 · y H 2 O with different values of the superconducting and magnetic-ordering temperatures, T c and T M , respectively. From measurements of the temperature and sample dependence of the NQR frequency, it was revealed that the NQR frequency is changed by the change of the electric field gradient (EFG) along the c axis ν z z rather than the asymmetry of EFG within the a b -plane. In addition, it is considered that the change of ν z z is gaverned mainly by the trigonal distortion of the CoO 2 block layers along the c axis, from the relationships between ν z z and the various physical parameters. We found the tendency that samples with ν z z larger than 4.2 MHz show magnetic ordering, whereas samples with lower ν z z show superconductivity. We measured the nuclear spin–lattice relaxation rate 1/ T 1 in these samples, and found that magnetic fluctuations depend on samples. The higher-ν z...Co nuclear-quadrupole-resonance (NQR) measurements were performed on various bilayered hydrate cobaltate Na_x(H_3O)_zCoO_2\cdot yH_2O with different values of the superconducting and magnetic-ordering temperatures, T_c and T_M, respectively. From measurements of the temperature and sample dependence of the NQR frequency, it was revealed that the NQR frequency is changed by the change of the electric field gradient (EFG) along the c axis \nu_{zz} rather than the asymmetry of EFG within the ab-plane. In addition, it is considered that the change of \nu_{zz} is gaverned mainly by the trigonal distortion of the CoO_2 block layers along the c axis, from the relationships between \nu_{zz} and the various physical parameters. We found the tendency that samples with \nu_{zz} larger than 4.2 MHz show magnetic ordering, whereas samples with lower \nu_{zz} show superconductivity. We measured the nuclear spin-lattice relaxation rate 1/T_1 in these samples, and found that magnetic fluctuations depend on samples. The higher-\nu_{zz} sample has stronger magnetic fluctuations at T_c. From the relationship between \nu_{zz} and T_c or T_M, we suggest that the NQR frequency can be regarded as a tuning parameter to determine the ground state of the system, and develop the phase diagram using \nu_{zz}. This phase diagram shows that the highest-T_c sample is located at the point where T_M is considered to be zero, which suggests that the superconductivity is induced by quantum critical fluctuations. We strongly advocate that the hydrate cobaltate superconductor presents an example of the magnetic-fluctuation-mediated superconductivity argued in the heavy-fermion compounds. The coexistence of superconductivity and magnetism observed in the sample with the highest \nu_{zz} is also discussed on the basis of the results of our experiments.

Correlation between Superconducting Transition Temperature T_c and Increase of Nuclear Spin-Lattice Relaxation Rate Devided by Temperature 1/T_1T at T_c in the Hydrate Cobaltate Na_xCoO_2・yH_2O(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

We have performed Co-nuclear quadrupole resonance (NQR) studies on Na x CoO 2 · y H 2 O compounds with different Na ( x ) and hydrate ( y ) contents. Two samples with different Na contents but near...

Magnetic and Superconducting Properties in Single Crystalline Fe1+δTe1-xSex (

The spin-fluctuation effect in the Se-substituted single crystalline Fe 1+δ Te 1- x Se x ( x = 0, 0.05, 0.12, 0.20, 0.30, 0.33, 0.45, and 0.48; 0≤δ≤0.12) and the polycrystalline Fe 1.11 Se has been studied by the measurements of the X-ray diffraction, the magnetic susceptibility under high magnetic fields and the electrical resistivity under magnetic fields up to 14 T. The samples with x = 0.05, 0.12, 0.20, 0.30, 0.33, 0.45, and 0.48 show superconducting transition temperatures in the ranger of 10–14 K. We obtained their intrinsic susceptibilities by the Honda–Owen method. A nearly linear-in- T behavior in magnetic susceptibility of Se-rich superconducting samples was observed, indicating the antiferromagnetic spin fluctuations have a strong link with the superconductivity in this series. The upper critical field µ 0 H c2 orb for T →0 was estimated to exceed the Pauli paramagnetic limit. The Kadowaki–Woods and Wilson ratios indicate that electrons are strongly correlated in this system. Furthermore, the s...

x<0.50

We succeeded in synthesizing powder samples of bi layer hydrate sodium cobalt oxide superconductors Na x CoO 2 · y H 2 O with T c = 0–4.6 K by systematically changing the duration in a 75% relative...

) System

The family of the superconducting quasiskutterudites (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) features a structural quantum critical point at x(c)=0.9, around which a dome-shaped variation of the superconducting transition temperature T(c) is found. Using specific heat, we probe the normal and the superconducting states of the entire series straddling the quantum critical point. Our analysis indicates a significant lowering of the effective Debye temperature on approaching x(c), which we interpret as a result of phonon softening accompanying the structural instability. Furthermore, a remarkably large enhancement of 2Δ/k(B)T(c) and ΔC/γT(c) beyond the Bardeen-Cooper-Schrieffer values is found in the vicinity of the structural quantum critical point. The phase diagram of (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) thus provides a model system to study the interplay between structural quantum criticality and strong electron-phonon coupling superconductivity.